This invention relates to vestigial sideband (v.s.b.) modulators.
Vestigial sideband modulators are used in television transmitters for modulating a video signal onto an r.f. carrier so that the modulated carrier has a vestigial sideband.
A typical v.s.b. modulator is shown in simplified form in FIG. 1. An input baseband signal e.g. a video signal is mixed in mixer 1 with an intermediate frequency (i.f.) carrier wave signal produced in an oscillator 2 to produce an amplitude modulated double sideband signal. The upper part of the upper sideband is removed in v.s.b. filter 3 and the signal is mixed in mixer 4 with an r.f. signal produced in oscillator 5, the frequency of which differs from the desired r.f. transmission frequency by the i.f. frequency. The output of the mixer 4 is filtered by a bandpass channel filter 6 to remove the r.f. carrier and the upper sideband frequency w.sub.c +2w.sub.i.f..
If it is desired to transmit at a different frequency, both oscillator frequency 5 and the channel filter 6 must be changed. The latter presents a problem since the channel filter depends on both the carrier frequency and the i.f. frequency, and is a relatively expensive and specialist item to obtain.